This invention relates to improvements in neutral gray colored glass of the type having low luminous (visible light) transmittance, and in particular to reductions in the ultraviolet and infrared transmittances, thereby lowering the total solar energy transmittance. The type of dark gray glass to which the present invention relates is characterized by luminous transmittance less than 20 percent at a thickness of 0.219 inches (5.56 millimeters).
In the past, gray colored heat absorbing glasses often relied on the inclusion of nickel as a chief coloring agent. But avoiding the use of nickel is desirable because the presence of nickel during melting sometimes leads to the formation of nickel sulfide stones in the glass. Although the nickel sulfide stones are nearly invisible and cause no harm to the glass under normal conditions, the high coefficient of thermal expansion of nickel sulfide can cause thermally induced stresses sufficient to fracture a glass sheet having a nickel sulfide stone. This is a particular problem in applications where the glass is subjected to a tempering process in which the presence of nickel sulfide stones can produce an unacceptably high rate of thermal breakage during or subsequent to tempering. Some prior art gray glass having nickel as a major colorant also has the disadvantage of undergoing a color shift as a result of being thermally tempered. Accordingly, it would be desirable to produce a gray glass that has the combination of low luminous transmittance (less than 20 percent) and reduced ultraviolet and infrared transmittance without the use of nickel compounds.
The following is a typical prior art dark gray glass composition, in which nickel is relied on for the gray color:
SiO.sub.2 72.90 percent by weight PA0 Na.sub.2 O 13.70 PA0 K.sub.2 O 0.03 PA0 CaO 8.95 PA0 MgO 3.90 PA0 Al.sub.2 O.sub.3 0.10 PA0 SO.sub.3 0.27 PA0 Fe.sub.2 O.sub.3 0.060 PA0 CoO 0.015 PA0 NiO 0.095
The luminous transmittance (C.I.E. illuminant C) of the above glass is 14.4 percent, the total infrared transmittance is 52 percent, the ultraviolet transmittance is 70.4, and the total solar energy transmittance is 38 percent at a thickness of 0.219 inches (5.56 millimeters). Although the visible transmittance is quite low, it may be noted that the ultraviolet and infrared transmittances are disproportionately high. It would be particularly desirable to lower the ultraviolet transmittance for the sake of reducing its fading effect on fabrics and other materials in automobiles and building interiors as well its aging effect on plastics.
Another nickel-containing gray glass composition is disclosed in U.S. Reissue Pat. No. 25,312 (Duncan et al.). The transmittances of the glass of that patent are relatively high and the glass would be considered a light gray glass that is intended to absorb only a moderate amount of solar radiation.
Attempts have been made to produce nickel-free gray glass as shown in U.S. Pat. No. 3,723,142 (Kato et al.) and British patent specification No. 1,331,492 (Bamford). In both of these patents the glasses are relatively transparent and are not the type of low luminous transmittance glass to which the present invention is directed.
Another attempt at nickel-free gray glass is disclosed in U.S. Pat. No. 4,104,076 (Pons) where Cr.sub.2 O.sub.3 or UO.sub.2 is used in combination with CoO and Se to produce a gray color. Although broad ranges for the coloring agents are disclosed in that patent, all of the examples have colorant concentrations that are not characteristic of the dark type of glass involved here. This is confirmed by the transmittance curves shown in FIGS. 2 through 5 of that patent, where transmittances in the visible light portion of the spectrum are well above 30 percent, and above 40 percent to a substantial extent, for all of the examples illustrated. In particular, it should be noted that the upper end of the broadest Cr.sub.2 O.sub.3 range disclosed is 0.0200 percent, and the largest amount of Cr.sub.2 O.sub.3 actually used in any of the Pons examples is 0.0085 percent. The goal of that patent is merely to produce a glass having a total solar transmittance of less than 50 percent (col. 2, lines 5-6) at a thickness of 6.2 millimeters, whereas the objective of the present invention is to reduce the total solar energy transmittance below the typical commercial product level of 38 percent referred to above. The Pons patent shows no awareness of the desirable results that are attained when the Cr.sub.2 O.sub.3 concentration exceeds those disclosed.
U.S. Pat. No. 3,300,323 (Plumat et al.) also involves an attempt to produce gray glass without nickel. Instead of nickel, this patent's approach requires the inclusion of TiO.sub.2 and optionally MnO.sub.2, both of which present significant drawbacks. A glass composition having substantial amounts of TiO.sub.2 is not compatible with the float forming process, by which most flat glass is produced. This is because the TiO.sub.2 causes a yellow color to form when the glass comes into contact with molten tin in the float process. Glass containing MnO.sub.2 has a tendency to form brown coloration when exposed to ultraviolet radiation, thus making product uniformity difficult to maintain. Additionally, the plural valance states of manganese makes control of the oxidizing conditions in the glass melting operation very critical, which renders control of the color difficult in a manufacturing operation.
A nickel-free, dark gray glass containing iron, cobalt, and selenium is the subject matter of co-pending, commonly assigned U.S. Pat. application Ser. No. 215,191 filed on July 5, 1988, by James V. Jones now U.S. Pat. No. 4,873,206. The glass disclosed there does not include chrome, relying instead on a relatively high concentration of iron and control of the redox conditions during melting to drive a certain amount of the iron to the ferrous state. While this approach may permit attaining transmittance goals similar to the present invention and may yield a product suitable for some applications, producing desired neutral color characteristics can be more difficult with that approach, and in particular, the sensitivity of the color to the redox level renders it difficult to maintain consistent color characteristics in the product. Color consistency is particularly important to the architectural market where uniformity of appearance over a large area glazed with many pieces of glass is a concern.